a) Field of the Invention
The present invention generally relates to a highly reliable semiconductor device and its manufacturing method, and particularly to a silicon semiconductor device having a high breakdown voltage and reliability and its manufacturing method.
In this specification, an "interstitial oxygen concentration" is a concentration defined by OLD-ASTM (ASTM 79).
b) Description of the Related Art
As semiconductor devices, particularly integrated circuit devices with MOS elements, are scaled down or becoming finer, the channel length tends to be shortened. As the channel length becomes shorter, a stronger electric field is generated in the device if the same voltage is applied, and a so-called hot carrier effect occurs. Hot carriers in a high energy state may cause leak current or deteriorate an oxide film if they pass through the film by tunneling.
A noticeable method of improving the reliability of a thermal oxide gate insulating film of a MOS transistor on a silicon substrate, is to reduce impurities in the silicon substrate and improve its crystallinity.
A flash memory is expected to become a substitute for a magnetic storage device in the near future. However, as carriers are injected into a floating gate, this tunneling current deteriorates the gate insulating film. It has been long desired to develop a high quality silicon oxide film capable of suppressing the gate insulating film from being deteriorated by a tunneling current.
The following two methods are supposed to be effective for improving the performance of a silicon substrate.
(1) Optimizing the manufacturing conditions of a silicon single crystal ingot from which silicon substrates are sliced.
(2) Improving the quality of a surface layer of a silicon substrate sliced from an ingot by processing the substrate.
Both the methods have been found contributive to the improvement of reliability of a gate insulating film.
The second method (2) will be first discussed.
With the Czochralski method, silicon is melted in a crucible made of carbon whose surface is covered with quartz. A single crystal is grown from the surface of Si melt. The grown silicon single crystal contains oxygen atoms in an oversaturated state, the oxygen atoms being melted out from quartz of the crucible. Oxygen atoms in a Si crystal have a variety of merits such as raising the crystal mechanical strength and gettering poisonous impurities.
However, the electrical property of an integrated circuit device using a silicon substrate sliced from a silicon single crystal ingot containing oxygen atoms in an oversaturated state may be degraded by oxidation-induced stacking faults (OSF) or surface micro defects (SMD) caused by precipitation of oxygen atoms in the form of SiO.sub.2 (or SiO.sub.x) during heat treatments of manufacturing processes.
For example, during heat treatments of manufacturing processes for an integrated circuit device, interstitial oxygen atoms contained in a silicon substrate precipitate on the area near the silicon substrate surface in the form of SiO.sub.2.
Under this condition, "oxidation" for forming a gate insulating film on a silicon substrate surface becomes abnormal "oxidation" at the area with precipitations. Therefore, the crystal structure of the oxide film does not show a regular tetrahedron structure of SiO.sub.2 at the area with precipitations.
It is known that the breakdown voltage at the area without a regular tetrahedron structure is degraded. The lifetime of a gate oxide film formed at the area with precipitations is shortened.
It is also known that even if interstitial oxygen atoms in a silicon substrate of an integrated circuit device locally precipitate in the form of SiO.sub.x during heat treatments of manufacturing processes, the SiO. precipitations can be changed harmless by subjecting the Si substrate to a heat treatment at about 1200.degree. C. before the formation of a gate insulating film, because the SiO.sub.x precipitations are decomposed and oxygen atoms are captured or trapped again in the crystal lattice.
However, general manufacturing processes for an integrated circuit device have essentially various heat treatments at a temperature lower than 1200.degree. C. following the heat treatment at 1200.degree. C. At this later heat treatment, oxygen atoms in the silicon substrate precipitate in some cases again on the substrate surface in the form of SiO.sub.x.
As a result, the 1200.degree. C. heat treatment for decomposing local SiO.sub.x precipitations is not so effective.
The following two methods are generally supposed to be effective for reducing the amount of SiO.sub.x precipitations to be caused by a heat treatment at a temperature lower than 1200.degree. C.
(3) Lowering an interstitial oxygen concentration near the substrate surface to a saturated concentration or lower at the lowest temperature of the integrated circuit device manufacturing processes (bulk processes for heating the silicon substrate).
(4) Suppressing generation of grown-in defects which allow interstitial oxygen atoms precipitate and generation of inhomogeneous nuclei of impurities such as carbon other than oxygen melted out of a quartz coated carbon crucible, during the process of growing a silicon single crystal by the Czochralski method.
The method (3) has been introduced from an analogous reduction of a saturated solution theory. Even if a silicon substrate contains nuclei for precipitating oxygen in the form of SiO.sub.x, SiO.sub.x will not be precipitated if oxygen atoms are not in an oversaturated state.
The method (4) suppresses generation of precipitation nuclei for interstitial oxygen atoms. For example, a pulling speed of a silicon single crystal is made sufficiently low. For example, the average pulling speed is lowered from about 0.8 mm/min to about 0.4 mm/min.
Although these methods are effective for preventing precipitation of interstitial oxygen, they are disadvantageous over the Cottrell effect which getters harmful impurities such as transition metals in a silicon substrate by precipitation nuclei.
For realizing both the impurity gettering and the reduction of interstitial oxygen concentration, the intrinsic gettering (IG) method has been used conventionally.
FIGS. 9A to 9D are cross sectional views explaining the processes of gettering oxygen atoms in a silicon substrate by the IG method.
In these figures, reference numeral 21 represents a silicon substrate, reference numeral 22 represents interstitial oxygen atoms, reference numeral 23 represents oxide films, reference numeral 24 represents precipitation nuclei, and reference numeral 25 represents stacking faults.
The outline of the IG method will be explained with reference to FIGS. 9A to 9D.